The Basics | The Key Technology | The Assembly

The Basics: How do the CLASS telescopes work?

The Cosmic Microwave Background (CMB) is a type of electromagnetic radiation, which has both intensity and polarization. CLASS uses this polarization to map over 70 percent of the sky.

CLASS telescopes are unique. New technologies we have developed make CLASS stable and sensitive at a level necessary to achieve our scientific goals. Our one-of-a-kind measurement strategy involves mapping 70 percent of the sky with cross-checks against systematic errors from a premier site in the Atacama Desert of northern Chile.

The Key Technology: Modulation

The CLASS telescopes are uniquely built to measure the large-angle polarization patterns in the CMB created during Inflation. “Modulation” is one of our most important aspects of technology that makes this measurement possible.

To illustrate how CLASS uses modulation, we have created the following audio-frequency example. In this example, the signal is represented by a voice saying “The Cosmic Microwave Background Polarization”. When unmodulated, the signal is difficult to isolate from low-frequency noise. With modulation, the signal is encoded at a distinct (and comically higher) frequency. The ear can then distinguish the high-frequency modulated signal from the low-frequency noise.

CLASS similarly modulates the polarization signal of the CMB. This modulation is analogous to AM radio. In fact, AM stands for amplitude modulation, in which a radio station’s audio signal is modulated at a specific radio carrier frequency to isolate it from the noise and the signals of other stations. CLASS uses high-frequency amplitude modulation to isolate the CMB polarization signal from the lower frequency atmospheric and instrumental noise.

The Assembly: CLASS 38 GHz focal plane timelapse

CLASS maps 70% of the sky at a frequency of 38 GHz (corresponding to a wavelength of 7.9 mm) in the microwave region of the electromagnetic spectrum to provide the most sensitive map of the sky ever made at these frequencies. To achieve this, the CLASS team has developed novel technologies like wide-bandwidth, smooth-walled copper feedhorns (patent pending) and the first transition-edge-sensor (TES) bolometers at a very low frequency. Our low-temperature bolometers operate at a temperature of just 0.1 º C above absolute zero, where atoms stop moving altogether — these are the most sensitive microwave detectors available.

The video below shows the CLASS 38 GHz focal plane as it was built in stages. It shows the 36 individual detector chips, microfabricated on silicon wafers at NASA’s Goddard Space Flight Center, as they were assembled into the focal plane, along with associated readout electronics around the edges. Each detector chip and its associate feedhorn form a pixel for the 38 GHz camera and measure the polarization of the cosmic microwave background.